The present invention relates to a demodulating device which demodulates a composite PSK-PSK modulated wave subjected to a 2.sup.n (n being a positive integer)-phase PSK modulation by a main data signal and further subjected to a 2-phase PSK modulation by means of a subdata signal.
Recent developments in a digital transmission systems have been remarkable, and various types of practical circuits are already available in the market. The transmission systems have been modified for many requirements and a system which is applicable to general purposes as well as highly effective in operation is being investigated. One example of such a system has been proposed in the Japanese Patent Application Disclosure No. 142008/79 published on Nov. 5, 1979, whose applicants are NEC Corporation and Nippon Telegram and Telephone Public Corporation. This system, as set forth above, is a composite PSK-PSK modulation system which comprises a 2.sup.n -phase PSK modulated wave carrying a main data signal and a 2-phase PSK modulated wave carrying the subdata signal. According to this system, subdata signals can be effectively transmitted without affecting the bit error rate of the main data signals if the ratio of the data rate f.sub.2 of the subdata signal against the data rate f.sub.1 (f.sub.2 .ltoreq.f.sub.1) of the main data signal is set below a certain value. When the number of quantized phases of the main data signal is given as 2.sup.n (n=1, 2, . . . ) and the amount of phase shift .alpha. in the 2-phase PSK modulated wave of the subdata signal is given as .alpha.=.pi./2.sup.n, a demodulated subdata has the highest immunity to noise. The modulation output vector of the composite PSK-PSK modulated wave becomes identical with a 2.sup.n+1 -phase PSK signal.
For phase detection of such a modulated wave, conventional delay detection or coherent detection may be employed. Coherent detection, however, entails a disadvantage such as described below. That is, whereas the main data signal component must be 2.sup.n -phase phase-detected, 2.sup.n+1 -phase phase detection must be employed for the phase synchronizing circuit which recovers the reference carrier wave. As is well known, a 2.sup.n+1 -phase synchronizing circuit is provided with a stably locked-in phase point at every 2.pi./2.sup.n+1 radians, including 2.sup.n number of stable points of 2/2.sup.n+1 (2i+1)[i=0, 1, . . . , 2.sup.n-1 ] which are unnecessary for 2.sup.n -phase phase detection.
An example of conventional means for avoiding such an unnecessary phase locking-in has been disclosed by NEC Corporation in Japanese Patent Application Disclosure No. 133812/79 published Oct. 17, 1979. In the modulation systems of this device, the amount of phase shift by the subdata signal is selected as .alpha.&lt;.pi./2.sup.n and the vector arrangement of the modulated output is different from that of a 2.sup.n+1 -phase PSK wave. The phase synchronizing circuit of the composite PSK-PSK modulation system comprises a phase detector for phase detecting an input signal, a first means for regenerating a subdata signal having the data rate of f.sub.2, a second means for obtaining at least two orthogonal signals that are the outputs from the phase detector from which subdata signal components have been removed by using the reproduced subdata signal as a control signal, a third means for obtaining a phase error signal by 2.sup.n frequency-multiplying the output of the second means, and a voltage controlled oscillator which is frequency-controlled by the output of the third means. According to this system, unneeded phase locking-in can be avoided; however, the amount of phase shift .alpha. for the subdata signal must be selected to be .alpha.&lt;.pi./2.sup.n. This makes the conventional system defective in that the immunity of the subdata signal against noise is degraded from an optimum value by 20 log ##EQU1## dB.